That means that if one part of the W. 106th Street bridge failed, the whole thing could collapse.
But there's a big difference between the bridges, too. City officials say the Bloomington bridge is in no danger of falling anytime soon.
"This bridge is in really good shape," said Steve Segar, a city civil engineer who is lead inspector for 13 Bloomington-owned bridges. "It's a structurally sound bridge that needs maintenance, just like most other man-made things."
The city recently hired a consultant to design a preservation plan for the bridge. By next month, SRF Consulting Group is supposed to present data on bridge maintenance and improvement costs and life expectancy. By September, the city should have preliminary plans on what needs to be done, and by December those plans should be ready for state review.
The city wants to strip a one-inch layer of concrete decking from the bridge, overlaying it with two inches of new concrete.
Butyl rubber seals at either end of the bridge that allow it to expand in the summer and contract in the winter need replacement. If those seals don't work, Segar said, moisture and salt can penetrate and corrode steel beams. Preliminary cost estimates hover around $500,000, with the bridge closing for the work for perhaps a couple of months in the summer of 2009.
The four-lane, 209-foot-long span sits at the bottom of a big dip in 106th Street, stretching over Nine Mile Creek. When it was built, the bridge won an award from the American Institute of Steel Construction as the most beautiful short-span bridge erected during 1969. It was designed by Howard Needles Tammen & Bergendoff, now known as HNTB, an architecture and engineering firm with headquarters in Kansas City.
With simple slatted silver fencing along the sides, the bridge looks sleekly functional at street level. From below, where the rushing creek mutes traffic noise, the bridge looms overhead and the design issue, as described by Segar, is evident.
Just two massive beams support the bridge deck across the creek. Segar said the construction style is called "rigid K frame." Viewed from creek level, the two welded Ks are tipped on their right sides, with the long straight part supporting the decking and two short legs sitting on either side of the creek bank.
The steel is rusted brown, but that was intentional in its design and doesn't reflect deterioration, Segar said.
"It's a weathering steel that has a natural rust coating and is designed to complement the surroundings," he said.
About 11,000 vehicles a day drive over the bridge, though traffic is heavier during the school year. The city regularly checks the bridge, and it is inspected by the state every five years with a snooper rig that drops over the side.
After the Minneapolis bridge collapse it was inspected just a year after the regular state check. Welds were evaluated with special electronics.
"The bridge was OK," Segar said. "There was no change in any of the welds."
While engineers today probably wouldn't build a fracture-critical bridge when there are other choices, Segar said, the bridge is sound. It's also something of a poster child for the creek and its watershed district, often appearing in photos with water rushing over rocks in the foreground. Many people walk on trails along the creek that pass underneath the bridge.
"We want to preserve the bridge as best we can, in the most cost-effective way over the long term," Segar said.
Mary Jane Smetanka • 612-673-7380